Firefox Session Restoration – which loads tabs and windows from previous session – time has significantly improved by volunteer contributor Kevin Jones and the lazy session restore has been landed in Nightly (Firefox 55). The lazy restore feature has been enabled by default and can be controlled using the preference browser.sessionstore.restore_tabs_lazily.

To know how lazy restore performed over normal restore with don’t load tabs until selected enabled, you need to check the below numbers. Kevin has restored  a huge number of tabs from session such as 330, 502 and 1008 on Mac OS, Nightly build 20170317213149 (2017-03-28), and you can see the time taken by Firefox to restore tabs with lazy and normal restore.

The lazy session restore when compared with normal session restore 

330 tabs:

Normal: 11s
Lazy: 1.5s

502 tabs:

Normal: 24s
Lazy: 2.4s

1008 tabs:

Normal: 85s (with fans whirring)
Lazy: 5.3s

The measurements above reveal how much the feature has been improved. You can see the responsiveness and fast loading of tabs in Nightly if you’ve set browser to start with previous session tabs and windows.

Don’t think Firefox is behind the development of lazytabs, which Opera and Vivaldi implemented sometime back recently. Mozilla has optimized the session restore feature to make the hundreds of tabs to load in less time.

Do note Mozilla has made ‘Don’load tabs until selected’ as default and removed the preference from tabs options in Firefox version 48. You won’t see the startup performance improvements or the Lazy tabs won’t work if you’ve disabled tabs on demand i.e turned browser.sessionstore.restore_on_demand preference value to false.

Lazy Tab Loading comes to Vivaldi 1.5, Activated by default

Opera 41: Lazy Tab Loading Feature